Your question piqued my curiosity, and after waiting for a while for a response from someone else, I measured the LP of my Santa Cruz #3. It measured 8'2+". In seven or so nationals I've sailed in or photographed in Santa Cruz and San Francisco, I've never seen anybody use a #4. Typically, #1s are used in the first race of the day followed to a change to #3s in the second or third race when the wind gets over 15-16 knots. This would vary with crew weight on board and perhaps with the size and cut of the #1.

My impression is that #4s are mostly used for day sailing when there is a breeze or in offshore races when the wind is really blowing hard.

As Scott Sorbie suggested, a max-size #3 might make more sense on Lake St. Clair or other mainly light air venues???

Comments from San Francisco Bay sailors/sailmakers would be appreciated.